Add weekly and monthly scheduled billing for offerings

Offerings can now bill weekly (a pending payment 24h before each lesson)
or monthly (one payment on the 1st for that month's lessons), alongside
one-time and full-term. Applies to both private lessons and group classes.

- Offering: new `weekly`/`monthly` billing modes + `isScheduledBilling()`
- Booking/enrolment defer payment for scheduled modes; a single lesson
  booked after its due date has passed (e.g. an add-on in an already-billed
  month) is charged at booking instead
- ScheduledBillingRunner: daily WP-Cron scan generates due payments across
  four cases (private/group × weekly/monthly), deduped via lesson.payment_id
  and payments.period_key
- PaymentDueMailer: one consolidated itemised email per student per scan
- Notice batch: payments emailed together share a reference; the admin
  Payments queue groups them with a lump-sum total for e-transfer reconciliation
- Cancellation never voids a scheduled payment (Payment::isScheduled())
- Schema: us_payments gains due_date, period_key, notice_batch; USC_VERSION 1.2.0

composer test, composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Offering/Offering.php

@@ -20,12 +20,26 @@ class Offering {
 	public const BILLING_ONE_TIME  = 'one_time';
 	public const BILLING_FULL_TERM = 'full_term';
 
+	/** Billed 24 hours before each lesson, on a recurring schedule (see scheduled-billing.md). */
+	public const BILLING_WEEKLY = 'weekly';
+
+	/** Billed on the first of each month for every lesson that falls in the month. */
+	public const BILLING_MONTHLY = 'monthly';
+
 	/**
 	 * All valid billing modes.
 	 *
 	 * @var list<string>
 	 */
-	public const VALID_BILLING_MODES = [ self::BILLING_ONE_TIME, self::BILLING_FULL_TERM ];
+	public const VALID_BILLING_MODES = [ self::BILLING_ONE_TIME, self::BILLING_FULL_TERM, self::BILLING_WEEKLY, self::BILLING_MONTHLY ];
+
+	/**
+	 * Billing modes whose payment is generated later by the daily billing scan
+	 * rather than taken at registration.
+	 *
+	 * @var list<string>
+	 */
+	public const SCHEDULED_BILLING_MODES = [ self::BILLING_WEEKLY, self::BILLING_MONTHLY ];
 
 	/** Listed in the public catalogue; anyone with `book_lesson` may enrol. */
 	public const ACCESS_PUBLIC = 'public';
@@ -71,6 +85,14 @@ class Offering {
 		return self::ACCESS_INVITE_ONLY === $this->accessMode;
 	}
 
+	/**
+	 * Whether this offering's payment is deferred to the daily billing scan
+	 * (weekly / monthly) instead of being taken at registration.
+	 */
+	public function isScheduledBilling(): bool {
+		return in_array( $this->billingMode, self::SCHEDULED_BILLING_MODES, true );
+	}
+
 	/**
 	 * The last day on which a student may enrol in this group class. Defaults to
 	 * the first day of the class (`term_start`) when the instructor has not set an